{"id":17749,"date":"2025-03-13T12:02:11","date_gmt":"2025-03-13T12:02:11","guid":{"rendered":"https:\/\/nmtest.sm4tech.com\/?post_type=glossary&#038;p=17749"},"modified":"2025-03-13T12:02:29","modified_gmt":"2025-03-13T12:02:29","slug":"error-521","status":"publish","type":"glossary","link":"https:\/\/nmtest.sm4tech.com\/ru\/glossary\/error-521\/","title":{"rendered":"\u041e\u0448\u0438\u0431\u043a\u0430 521"},"content":{"rendered":"<p>When using <a href=\"https:\/\/nmtest.sm4tech.com\/ru\/\">residential proxies <\/a>to access websites or online services, encountering errors can disrupt your workflow. One such error is Error 521, which occurs when the origin server (the website\u2019s hosting server) refuses the connection initiated by Cloudflare, a widely used content delivery network (CDN) and security service.\u00a0 Error 521 is a web server down error that occurs when Cloudflare attempts to connect to the origin server, but the server refused the connection. This typically happens when the origin server is offline, misconfigured, or experiencing technical issues.<\/p>\n\n\n\n<p>The error message usually reads:<\/p>\n\n\n\n<p><strong>\u201cError 521: Web server is down.\u201d<\/strong><\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Why Does Error 521 Occur?<\/strong><\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Server Offline: <\/strong>The origin server may be completely down due to maintenance, hardware failure, or power outages.<\/li>\n<\/ul>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Firewall Restrictions<\/strong>: The server\u2019s firewall may be blocking Cloudflare\u2019s IP addresses, preventing a connection.<\/li>\n<\/ul>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Misconfigured Server<\/strong>: Incorrect server settings, such as port configurations or SSL\/TLS issues, can cause the server to refuse connections.<\/li>\n<\/ul>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Network Issues<\/strong>: Connectivity problems between Cloudflare and the origin server, such as routing issues or ISP outages, can lead to this error.<\/li>\n<\/ul>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Proxy Misconfiguration<\/strong>: If you\u2019re using a proxy, incorrect settings or low-quality proxies can contribute to connection issues.<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>How to Fix Error 521<\/strong><\/h2>\n\n\n\n<p><strong>1. Check the Origin Server<\/strong><\/p>\n\n\n\n<p>Verify if the origin server is online and functioning properly. Use tools like Ping or Traceroute to test the server\u2019s availability.<\/p>\n\n\n\n<p>Contact the website owner or hosting provider to confirm if there are any server-side issues.<\/p>\n\n\n\n<p><strong>2. Review Firewall Settings<\/strong><\/p>\n\n\n\n<p>Ensure that the server\u2019s firewall is not blocking Cloudflare\u2019s IP addresses.<\/p>\n\n\n\n<p>Whitelist Cloudflare\u2019s IP ranges to allow seamless communication. You can find Cloudflare\u2019s IP list on their official website.<\/p>\n\n\n\n<p><strong>3. Check Server Configuration<\/strong><\/p>\n\n\n\n<p>Verify that the server is configured correctly, including port settings and SSL\/TLS certificates.<\/p>\n\n\n\n<p>Ensure that the server is listening on the correct port (e.g., port 80 for HTTP or port 443 for HTTPS).<\/p>\n\n\n\n<p><strong>4. Test Network Connectivity<\/strong><\/p>\n\n\n\n<p>Check the network connection between Cloudflare and the origin server to identify any connectivity issues.<\/p>\n\n\n\n<p>If you\u2019re using a proxy, try switching to a different proxy server or location to see if the issue persists.<\/p>\n\n\n\n<p><strong>5. Restart the Server<\/strong><\/p>\n\n\n\n<p>If the server is experiencing temporary issues, restarting it may resolve the problem.<\/p>\n\n\n\n<p>Ensure that all necessary services (e.g., web server, database) are running after the restart.<\/p>\n\n\n\n<p><strong>6. Use a Different Proxy<\/strong><\/p>\n\n\n\n<p>If the issue persists, switch to a different proxy server or provider. High-quality proxies, such as those offered by NodeMaven, are less likely to cause connection issues. <a href=\"https:\/\/nmtest.sm4tech.com\/ru\/locations\/us-proxy\/\">US proxy <\/a>of NodeMaven are very popular on the market.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Preventing Error in the Future<\/strong><\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Use reliable, high-performance proxies from trusted providers like NodeMaven.<\/li>\n<\/ul>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Regularly monitor the health and performance of your origin server.<\/li>\n<\/ul>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Ensure proper configuration of both your proxy settings and the origin server.<\/li>\n<\/ul>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Implement redundancy by using multiple proxy servers or locations to minimize downtime.<\/li>\n<\/ul>\n\n\n\n<p>Error 521 is a connection refusal issue that can occur when using proxies, but it can be resolved by identifying and addressing its root causes. By checking the origin server, reviewing firewall settings, and ensuring proper proxy configuration, you can minimize the risk of encountering this error.<\/p>\n\n\n\n<p>For reliable and high-performance proxies that reduce the likelihood of errors like 521, consider NodeMaven and its <a href=\"https:\/\/nmtest.sm4tech.com\/ru\/proxies\/mobile-proxies\/\">\u043e \u043c\u043e\u0431\u0438\u043b\u044c\u043d\u044b\u0445 \u043f\u0440\u043e\u043a\u0441\u0438<\/a>. Their robust infrastructure and global IP pool ensure seamless connectivity and an enhanced online experience.<\/p>","protected":false},"featured_media":0,"parent":0,"template":"","class_list":["post-17749","glossary","type-glossary","status-publish","hentry"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.3 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Error 521: What it is and How to Avoid?<\/title>\n<meta name=\"description\" content=\"Find out all the necessary information about Error 521 with the help of the developed proxy glossary from NodeMaven\" \/>\n<meta name=\"robots\" content=\"noindex, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<meta property=\"og:locale\" content=\"ru_RU\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Error 521: What it is and How to Avoid?\" \/>\n<meta property=\"og:description\" content=\"Find out all the necessary 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